I like to think of the Constitution as a living document that continues to very slowly evolve to serve our society. I've never thought of it as elitist because it spells out rights for everyone, not just the elite. Although the difficult process of amending it does make it favor the status quo.

The fact that the Constitution was written "without authority" is meaningless because no one has any authority in a government that is just being created.  And as litteacher8 points out, it was ratified by the states.   The men who created the Constitution were "elitist" in the sense that they were intelligent, educated, and mostly wealthy men.  However, who else could have done this job, a rabble of illiterate people?  If we view the events of the past week in the Middle East, we can see the consequences of the judgement and actions of a collection of uneducated people without resources.  

The Constitution was contemplated as a living document.  It is a mere eight pages long, and the framers contemplated that there would not only be amendments, but also that it was a document that would require continuing interpretation.  They realized that these eight pages could not possibly cover all eventualities and that times and the country would change.  Every time a court relies on the Constitution to make a decision, new life is being breathed into it! 

The history of the Constitution of the United States just proves what a remarkable document it is.  How it was written or who wrote it is not really significant.  It WAS ratified.  It has also gotten our country this far, and kept us relatively successful.  The ability to amend the Constitution keeps it current, and thus relevant and binding.

These things are not really opposed to one another.  We can see the Constitution as all of these things.  In some way it was an elitist document.  At the same time, it was an expression of the political philosophy (republicanism as opposed to democracy) of those elites.  This does not prevent it from being a living document as it has changed to reflect our changing political ideals.

I'm sure that at the time it was written the Constitution could have been seen as an elicit document.  While it is true that the authors had no real authority, they certainly had the respect of the people.  They are typically viewed as leaders of the newly forming country rather than radicals scheming behind closed doors.  I would say the Constitution is most often viewed now days as a political philosophy.  Although, I supposed it could be considered a living document because it can still be changed and amended.  It certainly has a living element to it even today.